Folks, I would like to know if an effect like "players can't draw cards" prevents/cancel effects like "if you would draw a card, instead do X".
Because I wouldn't draw a card anyway, the effect does not takes place?
For a concrete example, think on Maralen of the Mornsong with Forbidden Crypt.
The ruling on Maralen seems to support your conclusion that nothing happens:
4/1/2008: While Maralen is on the battlefield, replacement effects that instruct a player to do something instead of drawing a card won't work.
Can't beats can. So when something tells you to draw a card (say the beginning of your draw step), and Maralen telling you that you can't draw cards you do not get to draw a card. Since you are not drawing a card the Crypt's replacement effect doesn't have a chance to apply.
101.2. When a rule or effect allows or directs something to happen, and another effect states that it can’t happen, the “can’t” effect takes precedence.
614.7. If a replacement effect would replace an event, but that event never happens, the replacement effect simply doesn’t do anything.
